The present invention relates to a semiconductor device. Particularly, the invention relates to a semiconductor drive substrate used for a light valve such as an active matrix liquid crystal display. More specifically, the invention relates to a structure where a pixel switching element group and a peripheral driver circuit are formed integrally on a common substrate.
Conventionally, there has been known a so-called TFT substrate in which thin film transistors (TFT) are integrally formed on an insulating substrate such as quartz so as to constitute a drive substrate of an active matrix light valve. For example, Japanese Patent Laid-open No. 3-101714 discloses a TFT substrate utilizing a polysilicon as a semiconductor thin film material. Further, there has been known another TFT substrate utilizing an amorphous silicon in place of the polysilicon. However, the polysilicon and amorphous silicon have a relatively small carrier mobility, hence it is difficult to form a peripheral drive circuit on the same substrate.
Recently, another technology has been developed and is disclosed in, for example, Japanese Patent Laid-open No. 3-100516 where a single crystal silicon or monosilicon layer is laminated on an insulating substrate such as quartz to form the drive substrate. In contrast to the polysilicon and amorphous silicon, the monosilicon has a relatively great mobility, and therefore has an advantage in that a peripheral drive circuit of high performance and large capacity may be formed concurrently by utilizing regular IC fabrication process. However, a process temperature of the IC fabrication reaches up to 1150.degree. C., causing pending deformation of a wafer due to a difference of thermal expansion coefficients between the monosilicon layer and the insulating substrate, resulting in a drawback in that a desired yield rate cannot be obtained.
Another technology has been recently developed in which a drive substrate of the light valve is formed of a bulk monosilicon wafer, as disclosed, for example, in 8th International Workshop on Future Electronic Devices, Mar. 14-16, 1990, pages 81-84, Kouch-Ken, Japan. The regular LSI fabrication process can be applied to the bulk monosilicon wafer without a problem. However, the wafer must be transformed into a transparent substrate for use in the light valve. An integrated circuit formed in the wafer is subjected to device transfer process twice, so as to replace the silicon wafer by a transparent substrate, thereby disadvantageously complicating fabricating steps.
In order to reduce the number of steps of the device transfer process in changing the bulk monosilicon wafer to become transparent, a prior art method utilizing epitaxial technology is disclosed, for example, in Japanese Patent Laid-open Nos. 63-90859 and 63-101831. In this prior art method, an insulating layer is formed on a surface of a single crystal silicon wafer, and thereafter a window is selectively opened to form therein an epitaxial monosilicon region. A transistor device is formed in this epitaxial region, and then the transistor device is transferred to a transparent substrate to thereby replace the silicon wafer. In the transferring process, the surface of the transistor device is coated by a protective film composed of silicon dioxide by CVD, and further the transparent substrate is laminated using an adhesive. Then, a bulk portion of the silicon wafer is removed while utilizing the above mentioned insulating film as an etching stopper.
In the last mentioned fourth prior art method, the regular IC fabrication process is applicable since the monosilicon wafer is utilized, and further advantageously a single device transfer process is needed. However, when transferring the integrated transistor device, the silicon wafer and the transparent substrate or a support member are laminated with each other by the adhesive. The adhesive normally contains contaminants such as alkali metal, hence the device surface is provisionally applied with a protective film or a passivation film. As described before, the protective film is composed of a silicon dioxide film containing phosphorus (PSG film) deposited by CVD. However, the adhesive contains water vapor and hydrogen gas besides the alkali metal, which may disadvantageously degrade electrical characteristics of the device. While the PSG film can effectively block contaminants such as alkali metal, the water vapor and hydrogen gas may permeate the PSG film, therefore failing to protect the device. In view of the problems of the prior art, a first object of the present invention is to prevent degradation of the device and to improve the reliability in the substrate structure where the light valve device formed on the monocrystal semiconductor layer is transferred to the transparent support member.
Further, in the above noted fourth prior art method, the surface of the monosilicon wafer is covered by the insulating film, which is then selectively opened to provide a window to form an epitaxial region for the integrated device. However, such a process is rather complicated and requires delicate control, and is therefore not suitable for mass production. In view of this, according to the invention, a drive substrate of the active matrix light valve is formed by using a substrate having a monosilicon layer disposed on a silicon wafer through an electrically insulative material (hereinafter, such a type of substrate is referred to as "SOI substrate"). Various disadvantages and drawbacks arise when a device element such as a transistor is formed in the monosilicon layer disposed on the electrically insulative material. These drawbacks include an increase in a leak current by a parasitic channel, an unstable substrate potential level, and the generation of a photoelectric leak current. Thus, a second object of the invention is to improve electrical characteristics of a transistor formed in the SOI substrate.